Flexographic printing is widely used in the production of newspapers and in the decorative printing of packaging media. Numerous photosensitive printing plate formulations have been developed to meet the demand for fast, inexpensive processing and long press runs.
Photosensitive printing elements generally comprise a support layer, one or more photosensitive layers, an optional slip film release layer, and an optional protective cover sheet. The protective cover sheet is formed from plastic or any other removable material that can protect the plate or photocurable element from damage until it is ready for use. If used, the slip film release layer is typically disposed between the protective cover sheet and the photocurable layer(s) to protect the plate from contamination, increase ease of handling, and act as an ink-accepting layer. After exposure and development, the photopolymer flexographic printing plate consists of various image elements supported by a floor layer and anchored to a backing substrate.
It is highly desirable that flexographic printing plates work well under a wide range of conditions. For example, the printing plates should be able to impart their relief image to a wide range of substrates, including cardboard, coated paper, newspaper, calendared paper, and polymeric films such as polypropylene. Importantly, the image should be transferred quickly and with fidelity, for as many prints as the printer desires to make.
Flexographic printing elements can be manufactured in various ways including with sheet polymers and by the processing of liquid photopolymer resins. Flexographic printing elements made from liquid photopolymer resins have the advantage that uncured resin can be reclaimed from the non-image areas of the printing elements and used to make additional printing plates. Liquid photopolymer resins have a further advantage as compared to sheet polymer in terms of flexibility, which enables the production of any required plate gauge simply by changing the machine settings.
Various processes have been developed for producing printing plates from liquid photopolymer resins as described, for example, in U.S. Pat. No. 5,213,949 to Kojima et al., U.S. Pat. No. 5,813,342 to Strong et al., U.S. Pat. Pub. No. 2008/0107908 to Long et al., and U.S. Pat. No. 3,597,080 to Gush, the subject matter of each of which is herein incorporated by reference in its entirety.
Typical steps in the liquid platemaking process include:                (1) casting and exposure;        (2) reclamation;        (3) washout;        (4) post exposure;        (5) drying; and        (6) detackification.        
In the casting and exposure step, a photographic negative is typically placed on a bottom glass platen and a coverfilm is placed over the negative in an exposure unit. The exposure unit generally comprises the bottom glass platen with a source of UV light below it (lower light) and a lid having flat top glass platen with a source of UV light above it (upper light).
Precise reproduction of the negative image detail onto the photopolymer requires that the negative be placed as close to the photopolymer layer as possible. All of the air is removed by vacuum so that any wrinkling of the negative or coverfilm can be eliminated. In addition, the bottom glass platen may be grooved to further remove any air between the coverfilm and the negative. Thereafter, a layer of liquid photopolymer and a backing sheet (i.e., a thin layer of polyester or polyethylene terephthalate) are cast on top of the coverfilm and negative to a predetermined thickness. A backing sheet, which may be coated on one side to bond with the liquid photopolymer, is laminated over the cast liquid photopolymer layer to serve as the back of the plate after exposure.
Upper and/or lower sources of actinic radiation (i.e., the upper and lower lights) are used to expose the photopolymer to actinic radiation to selectively crosslink and cure the liquid photopolymer layer in the areas not covered by the negative. The top sources of actinic radiation are used to create the floor layer of the printing plate (i.e., back exposure) while the bottom sources of actinic radiation are used to face expose the photopolymer to actinic radiation through the negative to create the relief image. Plate gauge may be set by positioning a top exposure glass at a desired distance from a bottom exposure glass after dispensing liquid photopolymer on the protected bottom exposure glass.
The upper light source is turned on for a prescribed amount of time to cause the photopolymer adjacent to the substrate to crosslink uniformly over the entire surface of the plate, forming the floor. Thereafter, areas to be imaged are exposed to actinic radiation from the lower light source (i.e., through the bottom glass platen). The actinic radiation shines through the clear areas of the negative, which causes the photopolymer to crosslink in those areas, forming the relief image that bonds to the floor layer. The liquid photopolymer that is not exposed to the lower light source (i.e., the uncured photopolymer) remains in a liquid state and can be reclaimed and reused.
The type of radiation used is dependent in part on the type of photoinitiator in the photopolymerizable layer. The digitally-imaged mask or photographic negative prevents the material beneath from being exposed to the actinic radiation and hence those areas covered by the mask do not polymerize, while the areas not covered by the mask are exposed to actinic radiation and polymerize. Any conventional sources of actinic radiation can be used for this exposure step. Examples of suitable visible and UV sources include carbon arcs, mercury-vapor arcs, fluorescent lamps, electron flash units, electron beam units, photographic flood lamps, and light emitting diodes (LEDs), in which the wavelength of emitted light is determined by the materials used in the active region of the semiconductor.
After the exposure is complete, the printing plate is removed from the exposure unit and the photopolymer that was not exposed to actinic radiation (i.e., the photopolymer covered by the negative) remains liquid and can be reclaimed for further use. In liquid platemaking, resin recovery is an important factor relating to the production of photopolymerizable resin printing plates because the resins used are relatively expensive. In all areas not exposed to UV radiation, the resin remains liquid after exposure and can be reclaimed. In a typical process, the uncured resin is physically removed from the plate in a process step so that the uncured resin can be reused in making additional plates. This “reclamation” step typically involves squeegeing, vacuuming or otherwise removing liquid photopolymer remaining on the surface of the printing plate. This reclamation step not only saves material costs of the photopolymer resin but also reduces the use and cost of developing chemistry and makes a lighter plate that is safer and easier to handle.
Any residual traces of liquid resin remaining after the reclamation step may then be removed by nozzle washing or brush washing using a wash-out solution to obtain a washed-out plate, leaving behind the cured relief image. Typically, the plate is placed into a washout unit where an aqueous solution comprising soap and/or detergent is used to wash away any residual unexposed photopolymer. The plate is then rinsed with water to remove any residual solution.
The cured regions of the printing element are insoluble in the washout (or development) solution, and so after washout, a relief image formed of cured photopolymerizable resin is obtained. The cured resin is likewise insoluble in certain inks, and thus may be used in flexographic printing.
Thereafter, the printing plate is subjected to various post exposure and detackification steps. Post exposure may involve submerging the plate in a water and salt solution and performing an additional exposure of the printing plate to actinic radiation (UV light) to fully cure the printing plate and to increase plate strength. The printing plate may then be rinsed and dried. The detackification step (if used) may involve the use of a germicidal unit (light finisher) to ensure a totally tack-free plate surface. This step is not require for all plates, as certain resins may be tack-free and thus printing press ready without the need for the detackification step.
Typically, the floor area on a traditional flexographic printing element prepared from a liquid photopolymer accounts for at least about half of the plate thickness, with the remainder of the plate thickness being the relief area. The floor gives dimensional stability to the plate and provides support to the relief area. However, because so much of the photopolymer is used up in making a floor that covers the entire sheet, a variation on this process has been developed that minimizes the floor area and significantly increases the amount of liquid photopolymer that may be reclaimed. These printing plates are commonly referred to as “imposition plates,” “island plates,” or “I-plates.”
I-plates are made by adding another step to the liquid platemaking process. Instead of making a floor that that extends over the entire plate, a second photographic negative is placed on top of the photopolymer layer. This negative (also referred to as a masking film) outlines the image areas on the negative. The plates are first exposed to the upper UV light from the lid through the masking film, causing islands of cured polymer to be formed beginning in the photosensitive layer adjacent to the substrate. The timing and intensity of the exposure are limited to prevent the polymerization extending all the way through the photopolymer layer from the substrate to the free surface of the layer. The second lower UV exposure, from below the relief image negative, then causes the cured detailed relief image to form on top of the islands thus created. This process is described, for example, in U.S. Pat. Pub. No. 2012/0082932 to Battisti et al. and U.S. Pat. Pub. No. 2014/0080042 to Maneira, the subject matter of each of which is herein incorporated by reference in its entirety. The relief image negative and masking film are aligned so that each image area of the relief image negative is approximately in the middle of each clear area of the masking film.
One of the difficulties in producing I-plates is in the registering of the masking film with the negative in the set up process so that the islands created in the floor area will line up with the desired relief image. The masking film is typically applied over the surface of the substrate and can slip, causing the masking film to mis-register. U.S. Pat. Pub. No. 2012/0082932 to Battisti et al. attempts to solve this problem by using a substrate that has been printed with a digital representation of the island image that is then adhered to the layer of liquid photopolymer.
Related application Ser. No. 14/693,062 to Vest et al., the subject matter of which is herein incorporated by reference in its entirety, describes another method of producing an I-plate in which a liquid photopolymer printing blank is selectively exposed to actinic radiation to create a relief image printing plate by scanning a light bar comprising a plurality of UV LEDs across an upper surface of the liquid photopolymer printing blank through the backing sheet to cure the layer of liquid photopolymer in selected areas, creating islands of cured polymer adjacent to the backing sheet. The light bar comprises a plurality of UV LEDs arranged across the width of the light bar so that as the light bar scans over the surface of the liquid photopolymer printing blank, the surface of the liquid photopolymer printing blank may be subjected to crosslinking and curing to create cured islands therein.
One of the more time consuming steps in the platemaking process is the creation of the masking film and/or the relief image negative. There remains a need in the art for a streamlined process for creating printing plates from liquid photoresins that allows one to proceed from plate to press more quickly.